The National September 11 Memorial opened in downtown Manhattan in 2011 to honor the 3,000 people who lost their lives in the terrorist attacks on September 11, 2001 at the World Trade Center, Shanksville, Pennsylvania, and the Pentagon, along with the six people who were killed in the World Trade Center bombings in February 1993.

We bring you the top 10 secrets of the memorial, ranging from the design, the costs, to the rules & regulations. When you’re done, take our newly launched walking tour, where we’ll delve into the Untold Stories of 9/11.

10. The 9/11 Memorial Has the Largest Man-Made Waterfalls in North America

The two reflecting pools are the center piece of the 9/11 Memorial’s design, a work titled “Reflecting Absence” by Michael Arad. The pools cover the exact footprints of the original Twin Towers. Each is about an acre in size and together they make the largest man-made waterfall in North America. The falls blast about 26,000 gallons of water a minute over the 30-foot, black granite walls. There was some concern that the waterfall might be too loud for the Memorial, but the president of the 9/11 Memorial, Joseph Daniels explained that the sound is calming, like the sound of the ocean. “It’s perfect,” he said. “Not overwhelming at all.”